chown: fails to set owner given as a UID number

[jfinkels]

The owner of a file can be set to a UID number, regardless of whether there is a named user in the /etc/passwd file with that UID. The uutils version of df currently fails with an error message in this case.

GNU chown:

$ touch f && sudo chown 12345 f
# succeeds with no output; owner of f is set to 12345

uutils chown:

$ touch f && sudo ./target/release/chown 12345 f
./target/release/chown: invalid user: '12345'

This is causing an error in the GNU test case tests/ls/nameless-uid.sh.